Optical communication systems are known in which optical signals carrying data are transmitted from a first node to a second or receive node over an optical fiber. Often the optical signals, each having a corresponding wavelength, are combined onto an optical fiber to provide a wavelength division multiplexed (WDM) optical signal. At the receive node, the optical signals in the WDM optical signal are optically demultiplexed and converted into corresponding electrical signals, which are then further processed.
Recently, optical communication systems have been deployed in which so-called “superchannels” are transmitted. Each superchannel includes a plurality of optical signals or a group of channels that are relatively close to one another in wavelength. Multiple superchannels can be transmitted and combined onto an optical fiber, and each superchannel may be routed or directed through the optical communication system as an individual channel or optical signal. At the receive node, instead of separating an incoming signal into individual channels, each having a single wavelength, the combined superchannels are separated into individual superchannels, each having a plurality of optical signals, and each optical signal having a corresponding one of a plurality of wavelengths.
Often the data carried by each optical signal in a superchannel may be separated in the electronic domain using coherent detection. Namely, a light source or laser, also referred to as a local oscillator, is provided at the receive node. The incoming superchannel, which, if polarization multiplexed, may be split by a polarization beam splitter (PBS) into two orthogonal signals having, for example, transverse electric (TE) and transverse magnetic (TM) polarizations, respectively. Each superchannel, one having a TE polarization and the other having a TM polarization, output from the PBS is combined with the light output from the local oscillator and may be passed through a 90-deg optical hybrid circuit. The optical hybrid circuit, in turn, outputs further optical signals to “balanced” photodetectors, which, in turn, generate corresponding electrical signals. The electrical signals, which are in analog form, may then be amplified by a transimpedance amplifier, and then converted to corresponding digital signals by analog-to-digital converter (ADC) circuitry for further processing.
As generally understood, balanced detectors include pairs of photodiodes, which generate corresponding electrical currents in response to the received optical signals from the optical hybrid circuit. The photodiodes are connected to one another in series in such a manner that the current generated by one is subtracted from the current generated by other. As a result, components of the generated currents associated with the noise in the local oscillator light as well as the non-selected optical signals in the superchannel are cancelled out. The resulting output from the photodetector advantageously includes an electrical signal having a frequency equal to or substantially equal to the difference between the selected optical signal in the superchannel and the local oscillator light. Accordingly, the output may be referred to as a “down converted” signal, which carries the data supplied by the transmitter, but at a frequency significantly less than that associated with optical frequencies. Such down converted signals may be readily demodulated to extract the data.
Balanced detection typically requires that the currents generated by each photodiode be “balanced”, i.e., that the DC (direct current) component of each be the same. For example, the photodiodes in the balanced detector should preferably be identical. Typically, however, due to non-idealities associated with the photodiodes, the optical hybrid circuit, and/or other components, such balanced detection may not be achieved. In other words, detection is “imbalanced”. That is, the DC component of the current generated by one photodiode does not equal that generated by the other photodiode in the balanced detector. For example, due to semiconductor processing variations, one photodiode may be larger than the other and thus may supply a disproportionate amount of current. As a result, the components of such current associated with the local oscillator light and the non-selected optical signals in the superchannel may be not cancel out.
Detector imbalance can increase the amount of interference due to the presence of other signals in the superchannel being supplied to the photodiodes. In particular, the ratio of channel to interference (C/I) is proportion to the following:2 dB+LO/desired signal−20 log 10(K)−10 log 10(1+number of adj chs)
In the above formula, LO is the intensity of the light output from the local oscillator, “desired signal” is the intensity of the selected optical signal to be detected, K is an amount of imbalance, and “number of adj chs” is the number of channels or signals in the superchannel. Thus, based on the above formula, by reducing “K”, C/I can be increased or improved and the data carried by the selected optical signal may be more accurately recovered. Alternatively, an effective gain can be realized in connection with such optical signal, thereby improving system performance.
There is a need, therefore, to provide a receive node that can improve the C/I ratio in the presence of detector imbalance.